ALBERT

Description: A case study is presented of a tropical low that formed near Darwin, Australia, during the monsoon and subsequently intensified over land. The study is based on European Centre for Medium Range Weather Forecast (ECMWF) analyses. Interpretations of the formation over the sea are given in terms of vorticity dynamics. The thermodynamic support for the intensification and maintenance of the low over land is investigated also. The analyses indicate that the intensification of the low depends on repeated bursts of deep convection occurring near the centre of the circulation that promote the further concentration of vorticity near the centre. This concentration of vorticity increases the local circulation about the centre, which amounts to increasing the local tangential wind speed and, through approximate gradient wind balance above the boundary layer, to a lowering of the central pressure. It is found that the horizontal transport of moisture into a mesoscale column centred on the low is approximately equal to the moisture lost by precipitation so that total precipitable water levels are not rapidly depleted over land. While the contribution to the overall moisture budget by surface fluxes is comparatively small, these fluxes are necessary to maintain conditionally unstable conditions near the vortex centre so that deep convective bursts can continue to occur there, even when the system is located far inland.

Description: We investigate the hypothesized effects of a uniform flow on the structural evolution of a tropical cyclone using a simple idealized, three-dimensional, convection-permitting, numerical model. The study addresses three outstanding basic questions concerning the effects of moist convection on the azimuthal flow asymmetries and provides a bridge between the problem of tropical cyclone intensification in a uniform flow and that in vertical shear. At any instant of time, explicit deep convection in the model generates flow asymmetries that tend to mask the induced flow asymmetries predicted by a dry, slab boundary-layer model of Shapiro, whose results are frequently invoked as a benchmark for characterizing the boundary-layer induced vertical motion for a translating storm. In sets of ensemble experiments in which the initial low-level moisture field is randomly perturbed, time-averaged ensemble mean fields in the mature stage show a coherent asymmetry in the vertical motion rising into the eyewall and in the total (horizontal) wind speed just above the boundary layer. The maximum ascent occurs about 45 degrees to the left of the vortex motion vector, broadly in support of Shapiro's results, in which it occurs ahead of the storm, and consistent with one earlier more complex numerical calculation by Frank and Ritchie. The total wind asymmetry just above the boundary layer has a maximum in the forward right sector, which is in contrast to the structure effectively prescribed by Shapiro based on an inviscid dry symmetric vortex translating in a uniform flow where, in an Earth-relative frame, the maximum is on the right. This article is protected by copyright. All rights reserved.

Description: Analyses of thermodynamic data gathered from airborne dropwindsondes released from the upper troposphere during the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment are presented. We focus on two systems that finally became hurricanes Karl and Matthew , and one system ( Gaston ) that attained tropical storm status, but subsequently weakened and never redeveloped during five days of monitoring. Data for all events show that the largest values of total precipitable water are collocated with the surface trough and with values of convective available potential energy that seem high enough to support convective organization. These values coincide mostly with low values of convective inhibition. Vertical profiles of virtual potential temperature show little variability between soundings on a particular day, but the system means from day to day show a slight warming. In contrast, vertical profiles of pseudo-equivalent potential temperature, θ e , show much more variability between soundings on a particular day on account of the variability in moisture. In all systems, there was is a tendency for the lower troposphere to moisten, but in the non-developing system, the troposphere became progressively drier in the height range between approximately 2 and 9 km during the five days of observations. In the developing systems, the troposphere moistened. The most prominent difference between the non-developing system and the two developing systems was the much larger reduction of θ e between the surface and a height of 3 km, typically 25 K in the non-developing system, compared with only 17 K in the developing systems. Conventional wisdom would suggest that, for this reason, the convective downdraughts would be stronger in the non-developing system and would thereby act to suppress the development. Here we propose an alternative hypothesis in which the drier air weakens the convective updraughts and thereby the convective amplification of absolute vorticity necessary for development. Copyright © 2012 Royal Meteorological Society

Description: We examine aspects of the thermodynamic structure of mature Atlantic hurricane Earl (2010) based on airborne dropwindsondes released from the upper troposphere during the National Aeronautics and Space Administration (NASA), Genesis and Rapid Intensification Processes (GRIP) experiment. Vertical sounding profiles of the data raise questions concerning the relative roles of isothermal expansion and relative humidity increase in elevating the equivalent potential temperature of air parcels spiralling inwards to the eyewall convection region. The observational results obtained for two successive days of this category 4 hurricane show that the isothermal expansion effect leads to roughly one half of the increment in equivalent potential temperature for boundary-layer air parcels moving between the region outside the eyewall and the eyewall and eye region. Copyright © 2012 Royal Meteorological Society

Description: A simulation of Hurricane Katrina (2005) using the Australian Bureau of Meteorology's operational model for tropical-cyclone prediction (TCLAPS) shows that the simulated vortex vacillates between almost symmetric and highly asymmetric phases. During the symmetric phase, the eyewall comprises elongated convective bands and both the low-level potential vorticity (PV) and pseudo-equivalent potential temperature θ e fields exhibit a ring structure, with the maximum at some radius from the vortex centre. During this phase the mean flow intensifies comparatively rapidly, as the maximum acceleration of the mean tangential wind occurs near the radius of maximum mean tangential wind (RMW). In contrast, during the asymmetric phase the eyewall is more polygonal, with vortical hot towers (VHTs) located at the vertices. The low-level PV and θ e fields have monopole structures with the maximum at the centre. The intensification rate is lower than during the symmetric phase because the mean tangential wind accelerates most rapidly well within the RMW. The symmetric-to-asymmetric transition is accompanied by the development of VHTs within the eyewall. The VHTs are shown to be initiated by barotropic–convective instability associated with the ring-like structure of PV in the eyewall where the convective instability is large. During the reverse asymmetric-to-symmetric transition, the VHTs weaken as the local vertical wind shear increases and the convective available potential energy is consumed by convection. The weakened VHTs move outwards, similar to vortex Rossby waves, and are stretched by the angular shear of the mean vortex. Simultaneously, the rapid filamentation zone outside the RMW weakens, becoming more favourable for the development of convection. The next symmetric phase emerges as the convection reorganizes into a more symmetric eyewall. It is proposed that vacillation cycles occur in young tropical cyclones and are distinct from the eyewall replacement cycles that tend to occur in strong and mature tropical cyclones. Copyright © 2011 Royal Meteorological Society

Description: We examine the physical constraints that must be satisfied to allow for a steady-state tropical cyclone in an isolated environment, paying particular attention to the need to replenish absolute angular momentum exactly at the rate that it is consumed and to the vanishing of the spin-up function above the frictional boundary layer. We conclude that it is unlikely that these conditions will be met simultaneously and question whether globally steady-state tropical cyclone solutions have merit. The implications for previous studies are discussed.

Description: Radius–height cross sections of selected fields derived from the dropsonde data: (a) tangential velocity component, contour interval 5 m/s, shading indicated on the side bar in m/s, and absolute angular momentum, black lines, contour interval 5 × 105 m2/s; (b) temperature perturbation, contour interval 2 K (positive values), 1 K (negative values), shading indicated on the side bar in K; (c) radial velocity component, contour interval 3 m/s, shading indicated on the side bar in m/s; (d) equivalent potential temperature, contour interval 10 K, shading indicated on the side bar in K, and absolute angular momentum, black lines, contours as in (a); (e) relative humidity, contour interval 10%, shading indicated on the side bar in %; and (f) a zoomed‐in version of (d) at heights below 3 km. Analyses of dropsonde data collected in Hurricane Edouard (2014) just after its mature stage are presented. These data have unprecedentedly high spatial resolution, based on 87 dropsondes released by the unmanned NASA Global Hawk from an altitude of 18 km during the Hurricane and Severe Storm Sentinel (HS3) field campaign. Attempts are made to relate the analyses of the data to theories of tropical cyclone structure and behaviour. The tangential wind and thermal fields show the classical structure of a warm‐core vortex, in this case with a secondary eyewall feature. Additionally, the equivalent potential temperature field (θe) shows the expected structure with a mid‐tropospheric minimum at outer radii and contours of θe flaring upwards and outwards at inner radii. With some imagination, these contours are roughly congruent to the surfaces of absolute angular momentum. However, details of the analysed radial velocity field are quite sensitive to the way in which the sonde data are partitioned to produce an azimuthal average. This sensitivity is compounded by an apparent limitation of the assumed steadiness of the storm over the period of data collection.

Description: An analysis of numerical simulations of tropical low intensification over land is presented. The simulations are carried out using the MM5 mesoscale model with initial and boundary conditions provided by ECMWF analyses. Seven simulations are discussed: a control simulation, five sensitivity simulations in which the initial moisture availability is varied, and one simulation in which the coupling between moisture availability is suppressed. Changing the initial moisture availability adds a stochastic element to the development of deep convection. The results are interpreted in terms of the classical axisymmetric paradigm for tropical cyclone intensification with recent modifications. Spin up over land is favoured by the development of deep convection near the centre of the low circulation. As for tropical cyclones over sea, this convection leads to an overturning circulation that draws absolute angular momentum surfaces inwards in the lower troposphere leading to spin up of the tangential winds above the boundary layer. The intensification takes place within a moist monsoonal environment, which appears to be sufficient to support sporadic deep convection. A moisture budget for two mesoscale columns of air encompassing the storm shows that the horizontal import of moisture is roughly equal to the moisture lost by precipitation. Overall, surface moisture fluxes make a small quantitative contribution to the budget, although near the circulation centre, these fluxes appear to play an important role in generating local conditional instability. Suppressing the effect of rainfall on the moisture availability has little effect on the evolution of the low, presumably because, at any one time, deep convection is not sufficiently widespread.

Description: Widely-held arguments attributing the increasingly rapid intensification of tropical cyclones to the increasing “efficiency” of diabatic heating in the cyclone's inner core region associated with deep convection are examined. The efficiency, in essence the amount of temperature warming compared to the amount of latent heat released, is argued to increase as the vortex strengthens on account of the strengthening inertial stability. Another aspect of the efficiency ideas concerns the location of the heating in relation to the radius of maximum tangential wind speed, with heating inside this radius seen to be more efficient in rapidly developing a warm core thermal structure and, presumably, a rapid increase in the tangential wind. A more direct interpretation of the increased spin up rate is offered when the diabatic heating is located inside the radius of maximum tangential wind speed. Further, we draw attention to the limitations of assuming a fixed diabatic heating rate as the vortex intensifies and offer reasons, on these grounds alone, why it is questionable to apply the efficiency argument to interpret the results of observations or numerical model simulations of tropical cyclones. Moreover, since the spin up of the maximum tangential winds in a tropical cyclone takes place in the boundary layer and the spin up of the eyewall is a result of the vertical advection of high angular momentum from the boundary layer, it is questionable also whether deductions about efficiency in theories that neglect the boundary layer dynamics and thermodynamics are relevant to reality.

Description: The recent study of the sensitivity of tropical-cyclone intensification to the surface drag coefficient in a three-dimensional model by Montgomery et al. is extended to include a wind-speed-dependent drag coefficient and one of four boundary-layer parametrization schemes: the bulk, Blackadar, MRF and Gayno–Seaman schemes. The schemes are slightly modified to have the same drag coefficient formulation and the same constant exchange coefficients for sensible heat and moisture. Interest is focussed on the change in intensity of the azimuthally-averaged tangential wind speed and change in the low-level vortex structure when the standard value of the drag coefficient is halved or doubled. Changing the drag coefficient provides insight into unbalanced effects in the boundary layer and their impact on the vortex evolution and structure. The changes in vortex behaviour with changing drag coefficient are qualitatively similar for all schemes, the maximum intensification occurring for a value somewhere near the standard value of the drag coefficient. The interpretation given to explain this behaviour underlines the intrinsically unbalanced nature of the boundary-layer dynamics, although, for reasons discussed, a complete theory for the behaviour does not exist. The behaviour found is at odds with the predictions of Emanuel's (balance) theory for the maximum intensity of a tropical-cyclone, which predicts a monotonic decrease in intensity with the drag coefficient if the enthalpy exchange coefficient is held fixed. It is at odds also with a recent numerical study of the maximum intensity by Bryan and Rotunno. The study underscores the importance of boundary-layer dynamics in models for forecasting tropical-cyclone intensity and the need for care in choosing a boundary-layer scheme. However, it is not yet known which boundary-layer formulation is the most appropriate for this purpose, highlighting the need for a concerted research effort in this direction. Copyright © 2012 Royal Meteorological Society